During cementation, plugs are lowered into a well bore. Once the cementation operation is completed these plugs, together with the associated float collar and/or float shoe are drilled out.
One of the difficulties in drilling out plugs is that they can rotate with the drill thereby seriously delaying progress.
Various proposals have been made for inhibiting the rotation of plugs in the well bore during drilling out. Typically, these comprise providing the plug and/or the float collar/shoe with anti-rotation devices which are intended to inhibit rotation of the plug relative to the float collar/shoe which is non-rotatably set in the casing.
Early anti-rotation devices included protrusions which had the disadvantage that the full weight of the drill string was applied to the protrusions via the plug with the result that the protrusions often broke under the axial load and were rendered largely ineffective.
Applicants WO-91/17340 discloses an anti-rotation device in which the axial load is carried by circular load bearing seals mounted on the upper surface of the float collar and the lower surface of the plug and both the plug and the float collar are provided with teeth which inter-engage but are not subject to any axial load. As a result, the teeth are used solely to transmit rotational forces.
Whilst this arrangement works generally acceptably it has a disadvantage that in deviated wells the protrusion on, for example the plug may land on the load bearing member of the float collar rather than inter-engaging the teeth thereof. On attempted rotation of the plug these protrusions may then be broken off.